Abstract
We investigated the leaching process of arsenic from Miocene sedimentary rocks (the Kawabata and Karumai Formations) and the change in leaching amount with weathering. The Kawabata and Karumai Formations show same clay mineral assemblage, smectite, chlorite and illite. Smectite is the dominant clay mineral in the Kawa-bata Formation, whereas chlorite and illite are dominant in the Karumai Formation. The leaching amount of arsenic shows an abrupt increase when the leachate pH exceeds 8.5. This leaching behavior suggests that arsenic adsorbed on ferric oxide and hydrous ferric oxide starts to leach with increasing pH of the leachate. The leachate pH is elevated by cation exchange between interlayer Na+ ions in smectite and H+ in the leachate. The high smectite content in the Kawabata Formation means that the leachate pH is high (up to 10.3) compared with that in the Karumai Formation (pH < 8.8). As a result, the leaching amount of arsenic from the Kawabata Formation (up to 0.074 mg/L) is larger than that from the Karumai Formation (< 0.006 mg/L). The exchangeable cation in smectite is mainly Na+ in unweathered rocks, whereas it in weathered rocks consists of other cations, such as Ca2+ and H+. Because the cation exchange capacity of Ca-smectite is smaller than that of Na-smectite, the cation exchange reaction is hard to occur between the leachate and weathered rocks. Consequently, the leachate pH does not rise and the leaching amount of arsenic is relatively small in the case of weathered rocks.
